Here's why:
1) Because a real life crisis is NOT like shooting a still target at the range, the target moves and can shoot back, aiming for center mass gives you the best chance of hitting them at all.
2) Because if the perp has a weapon they could shoot you dead even after you "blew out their kneecaps". If the perp is on drugs they may feel no pain and beat you to death with your own weapon.
3) Because the police do not "shoot folks in the leg" for a reason, they practice a whole lot more than civilians do, they have better aim and a LOT more training which is why they shoot multiple times to drop the target as fast as possible.
4) Because using a firearm to stop a threat/attacker is a brutal and often very bloody endeavor, if you are going to have a gun for protection then you need to accept that reality. If you are afraid to inflict serious bodily injury then you are best off using mace or a stun gun.
5) Because if they are dead then they won't be able to tell their side of the story in court.
(I am sure there are other reasons that I didn't think of and someone else can chime in.)